This invention relates to magnetic alloys for use in thin film magnetic recording media for horizontal recording.
Cobalt-based alloys are widely used for horizontal magnetic recording in thin film magnetic disks. The cobalt-based alloys are typically deposited by plating, sputtering or vacuum deposition. Typical alloys used in films of magnetic media include Co-Ni, Co-Ni-Cr, Co-Cr-Ta, Co-Pt, Co-Ni-Pt, Co-Cr-Pt, Co-Sm and Co-Re. Magnetic alloys used for thin film media must generally satisfy the following criteria. First, the alloy coercivity must be sufficiently high to support a high recording density. Second, the saturation magnetization must be high to produce a strong output signal for a given film thickness. Third, the signal to noise ratio provided by the magnetic alloy should be high. Fourth, the alloy should exhibit as a high corrosion resistance as possible. Fifth, alloy selection should minimize manufacturing costs. Optimization of these criteria is extremely difficult.
As mentioned above, to support a high recording density, the coercivity Hc must be high enough to sustain a high density of flux reversals per linear inch. For typical high density applications, coercivity is 1200 Oe or higher, and it is projected that much higher coercivity may be required by the year 2000 as described by M. Kryder in Data Storage in 2000--Trends in Data Storage Technologies, IEEE Trans. on Magnetics, (November, 1989), incorporated herein by reference. Also see T. Yogi et al., "Longitudinal Media for 1 Gb/in.sup.2 Areal Density", Research Report, published by IBM, Apr. 17, 1990. Thus, providing a high alloy coercivity is extremely important.
In order for the media to provide a strong output signal during read-back, the media should have an intrinsically high saturation magnetization Ms greater than or equal to approximately 400 emu/cc and preferably 600 emu/cc or higher. The media should also have a high hysteresis loop squareness S (S=Mr/Ms) of at least 80% to provide a high magnetic remanent Mr. The strength of the read-back signal received from the read-write head (in the case of a single element inductive head where the same coil is used for both reading and writing) is proportional to film thickness T times the magnetic remanent Mr. Mr.times.T should generally be greater than or equal to 1.times.10.sup.-3 emu/cm.sup.2, and usually greater than 2.times.10.sup.-3 to 3.times.10.sup.-3 emu/cm.sup.2 to provide a sufficiently strong output signal for a practical disk drive. If one can achieve a high magnetic remanent Mr, one can use a thinner alloy film, thereby using less magnetic alloy in the manufacturing process, and thus having lower manufacturing costs. Also, by forming thinner alloy films, the sputtering apparatus may be opened up and cleaned less frequently.
The above-described magnetic parameters are described in greater detail in the article "Thin Film Magnetic Recording Technology: A Review" by J. K. Howard, published in the Journal of Vacuum Science and Technology, January 1986, incorporated here and by reference.
Co-Pt alloys are well known in the art, and exhibit high saturation magnetization and can exhibit high coercivity. Co-Pt alloys have been described, for example, by Opfer et al. in "Thin Film Memory Disk Development", Hewlett-Packard Journal, November 1985, pages 4-10, Aboaf et al., in "Magnetic Properties and Structure of Cobalt-Platinum Thin Films", IEEE Trans. on Magnetics, pages 1514-1519, July 1983, and U.S. Pat. No. 4,438,066, issued to Aboaf et al., each incorporated herein by reference. The Aboaf article indicates that up to a Pt concentration of 25 atomic percent coercivity increases as Pt content increases. (See Aboaf article FIG. 3 and the accompanying text on page 1515.) (Hereinafter, the abbreviation at. % is used to refer to atomic percent.)
One problem with Co-Pt alloys is their lower resistance to corrosion. It is generally known to add Cr to a Co-Pt alloy to form a ternary alloy in order to enhance corrosion resistance. U.S. Pat. No. 4,789,598 issued to Howard, et al. incorporated herein by reference, advocates a Cr content of 17 at. % to provide corrosion protection (and to reduce noise). Japanese laid open patent application 59-88806, filed by Yanagisawa et al., incorporated herein by reference, describes an experiment in which a number of disks, including Co-Cr-Pt alloys with a Cr content between 6 and 17 at. % were immersed in water at 25.degree. C. It was claimed by Yanagisawa that there was no loss in saturation magnetization (a symptom of corrosion) as a result. (See Yanagisawa FIG. 4.)
We performed our own experiment on disks including ternary alloys of Co, Cr and Pt alloys with a Pt content from 10 to 11 at. % and a Cr content varying from 6.5 to 13.2 at. %. The disks included an Al substrate, a Ni-P plated underlayer, a Ni-P sputtered underlayer, the Co-Cr-Pt alloy, and a 35 nm thick carbon overcoat. The disks were immersed in water at 80.degree. C. for 24 hours. We discovered that Cr adequately protected the ternary Co-Cr-Pt alloys if the Cr content was at least 10 at. %. The ratio of final to initial saturation magnetization Ms compared with Cr content is graphed in FIG. 1. This experiment uses more aggressive conditions than Yanagisawa to show the effect of Cr as corrosion protection, and demonstrates that the alloy's ability to resist corrosion increases markedly at a Cr content of about 10%.
Although Cr is advantageous in that it retards corrosion, addition of Cr is also disadvantageous in that it decreases alloy saturation magnetization Ms. FIG. 2 illustrates saturation magnetization 4.pi.Ms versus Cr content for a binary Co-Cr alloy in bulk form. (4.pi.Ms is measured in units of Gauss.) (Cr similarly reduces the saturation magnetization Ms of Co-Pt alloys.) As can be seen, 4.pi.Ms drops rapidly, such that if Cr equals about 25 at. %, 4.pi.Ms drops to essentially zero. At a Cr content of 10 at. %, 4.pi.Ms drops from about 18,000 to 11,000 Gauss. The data in FIG. 2 is similar to the behavior for thin film Co-Cr, e.g. as described in FIG. 1 of "Co-Cr Recording Films With Perpendicular Magnetic Anisotropy" by Iwasaki et al., IEEE Trans. Mag. Vol. Mag-14, No. 5, Sept. 5, 1978, pages 849-851, incorporated herein by reference. Thus, if one wanted to retard corrosion in an alloy by adding Cr, but wanted to retain a constant value for the parameter Mr.times.T, adding Cr would mean that thickness T would have to be increased. As mentioned above, this would make the media more expensive. Further, making the magnetic alloy thicker degrades the recording performance and recording density. Thus, Cr addition to retard corrosion should be minimized if possible to avoid degrading Ms.
As indicated above, it is also necessary to minimize noise in magnetic media. U.S. Pat. No. 4,789,598, issued to Howard et al., indicates that noise can also be reduced in a Co-Pt alloy by adding Cr such that the Cr concentration is greater than at. 17%. As can be seen in FIG. 2, addition of 17 at. % Cr to pure Co would drop 4.pi.Ms to about 6000 Gauss. Based on the information contained in Howard FIG. 4, one can calculate that Howard experiences a 4.pi.Ms of 3973 Gauss for his ternary Co-Cr-Pt alloy. Such saturation magnetization values are undesirably low. (The discrepancy between the 6000 Gauss predicted by FIG. 2 and the 3972 Gauss predicted by Howard FIG. 4 might be because 1) Howard's alloy contains Pt and FIG. 2 provides data for alloys lacking Pt, and 2) Howard's data is based on a thin film alloy, and FIG. 2 is for bulk Co-Cr.)
In order to obtain the optimal combination of high coercivity, saturation magnetization, corrosion resistance, low noise, and low production costs, selection of the magnetic alloy and selection of alloy composition range are key factors. High Pt content is necessary to increase coercivity, but adds to the cost of the fabrication. Cr content can be increased to enhance corrosion resistance. However, as mentioned above, Cr greatly reduces saturation magnetization Ms. Thus, the Cr content must be minimized to provide a sufficiently high saturation magnetization Ms while still adequately protecting against corrosion.